Data storage is an important aspect of today's information technology. A great deal of effort has been made by the storage industry to increase the real data density of a storage medium in order to meet the ever increasing demand for higher capacity storage devices.
Magnetic storage devices such as fixed or removable magnetic disks and tapes are widely-used conventional storage devices. The state-of-art conventional magnetic hard drive systems can achieve extremely high linear bit densities, especially with the new MR and GMR magnetic heads. For example, the real density of many hard disk drives is on the order of magnitude of about one gigabit per square inch. One limitation in increasing real data density in a magnetic device is the particle size or the characteristic dimension of a typical magnetic domain of the magnetic recording materials. Other limitations include the width of the magnetic read/write head and the limitations of mechanical tracking. Therefore, these hard drives are typically limited to less than 10,000 tracks per inch.
Optical storage devices are emerging as an alternative technology to the conventional magnetic technology because of their potential for high density data storage. The real density of an optical storage device, in principle, is only limited by the diffraction limit of an illuminating optical beam for reading or writing. One type of commercial optical storage technology is based on magneto-optical materials. These materials can currently produce an real data density of about one giga bit per square inch.
One well-known approach to increase the real data density in an optical storage system is using smaller beam size. Due to the diffraction limit, this may be achieved by using a light source with shorter wavelengths such as those toward the blue end of the spectrum. For example, one application for the industrial development of compact blue lasers is aimed at the optical storage. Alternatively, one may increase the numerical aperture of the objective lens in the system to focus a beam at a given wavelength to a smaller spot within the diffraction limit.
FIG. 1 shows a block diagram of a typical rewritable optical storage system or drive 100 based on magneto-optic recording in the prior art. Several servo mechanisms are needed to keep the laser beam in focus and tracking on the disk, for example, an objective lens actuator 114, a master-slave tracking servo control 130, and a focusing servo control 120. In particular, the objective lens in the prior-art system 100 is servo controlled for focusing and tracking the beam onto the storage medium layer(s) at a desired location. This type of conventional optics system is usually limited to numerical apertures of the objective lens of less than 1.0, and typically in a range about 0.55 to 0.60. Since the areal density of the data stored on the medium is directly proportional to the square of the numerical aperture, the limited numerical apertures of a conventional optical drive can significantly restrict a substantial increase in the data density.